The present invention relates to an electronic timepiece having an electro-optic passive display cell. The timepiece comprises an oscillator used as a time base and an electronic control circuit for the several functions of the timepiece. The control circuit is operable by means of at least one stationary key or switch having a capacitive action which is carried by the glass or crystal of the timepiece and which is fed, at least indirectly, by the oscillator through a fixed condenser.
Such timepieces are known and FIG. 1 shows diagrammatically the section of a portion of one of such timepiece, that is to say a watch provided with a display cell having liquid crystals. This cell is composed, mainly, of a rear plate 1 and of a front plate 2, separated from each other by a frame 3. These plates carry electrodes 4 and 5 respectively, which are intended to create an electric field in the space which separates them, and which space is filled with a liquid crystal 6.
The glass or crystal, designated by 7, of this watch, covers the whole and carries, on its inner face, an electrode 8, forming a stationary key, which constitutes one of the plates of a variable condenser 9, shown in FIG. 2, which figure represents a block diagram of the watch. The other plate of condenser 9 is constituted by the finger of the user, represented at 10, which is put to ground of the watch by the intermediary of the bottom of the casing of the watch. Ground is also connected to one of the poles of the source of voltage, not represented, feeding all the circuits of the watch.
The block diagram of FIG. 2 represents the conventional circuits of the watch, that is to say an oscillator 11, used as a time base, and an electronic circuit 12 comprising the frequency dividers and all the annexed circuits such as the resetting circuit, the alarm circuit, the circuit of chronograph, etc. . . . This circuit 12 delivers, especially, the signals which are necessary for the display circuit 13 to display the time information.
The oscillator 11 delivers also a signal, through a separator stage 14, to a capacitive divider comprising a fixed condenser 15, which is integrated into the circuit of the watch or at least, mounted in proximity thereof as a discrete element and which is connected in series with the variable condenser 9 constituted, as disclosed hereabove, by the electrode 8 and the finger 10 of the user. Consequently, condenser 9 has a non-negligible capacity only when the user puts his finger to the crystal 7 opposite the electrode 8. The rest of the time, its capacity is very low and its impedance is high.
The point 16 which is common to the two condensers 9 and 15 is connected to the input 17a of a comparator circuit 17 which receives, on a second input 17b, a reference signal delivered by a circuit 18 which is also connected to the output of the separator 14.
The comparator 17 is arranged so as to deliver at its output 17c a logical signal, used for the control of the several functions of the electronic circuit 12. The output 17c takes, for instance, the logic states 0 to 1 according to whether the signal applied to the input 17a has an amplitude greater or smaller than that of the reference signal, that is to say whether the impedance of the variable condenser 9 is great or small, or still whether the user does not put or does put his finger onto the stationary key.
The electrodes 4 of the display cell are connected, in a not shown way, to the ground of the watch. The electrodes 5 are, on their side of the liquid crystal connected, by the control circuit of the display to one or the other of the poles of the votage source according to the information which has to be displayed. These electrodes 4 and 5 constitute consequently, with the electrode 8 of the stationary key, a parasitic variable condenser 19, represented in dotted lines in FIG. 2. This parasitic condenser 19 is connected in parallel with the condenser 9 and reduces the margin of safety existing between the activated and non-activated states of the stationary key.
The purpose of the present invention is to remove the influence of this parasitic condenser.